This invention relates in general to running casing hangers for offshore wells, and in particular to a tool that provides an indication that a subsea casing hanger has properly landed and provides a hold down for the casing hanger and casing string while cement is being pumped through the casing string.
In one type of offshore drilling, a wellhead housing is located at the sea floor. One or more strings of casing extend in the well, each being supported by a casing hanger in the wellhead housing. The casing hanger is run on a running tool, which also sets a seal between the casing hanger and the wellhead housing after the casing has been cemented.
In the Gulf of Mexico, a shallow formation that must be drilled through often requires an additional casing string over what would normally be used. In one technique, the wellhead assembly includes a sub that extends below the wellhead housing. A first string of casing extends downward from the sub. A landing shoulder is provided in the sub for landing a casing hanger for a second string of casing. It is important that the casing hanger land on the landing shoulder before cementing occurs. Also, the casing hanger must remain on the landing shoulder during cementing. The cement returns flowing up the casing armulus tend to float the casing hanger upward.
The wellhead assembly has a profile located above the landing shoulder. The running tool assembly includes a hold down mechanism as well as a running mechanism. The hold down mechanism has a body mounted above the running mechanism. A plurality of dogs are mounted to the body for radial movement. After the casing hanger lands, the dogs are moved to an engaged position with the profile. The operator is able to pull upward on the running string to verify that the dogs are in engagement with the profile. While cementing, the cement returns flow past the dogs, with the dogs resisting any upward movement of the casing hanger.
In the preferred embodiment, the dogs are actuated by straight downward movement of the running string after the casing hanger has landed. The hold down mechanism has a mandrel with a cam surface that moves downward with the running string. A latch member between the mandrel and the hold down mechanism resists upward movement of the mandrel. The mandrel and a lower portion of the body are rotatable relative to theupper portion of the body, which holds the dogs. This allows the operator to rotate the running string, with the rotation transmitting through the mandrel and lower portion of the body to the running mechanism.